1 review

Sarah Leigh (Genomics England Curator)

Green List (high evidence)

Vaz et al. (2019 - PMID: 31637422 - DDD study among the co-authors) report on 5 individuals - from 4 families - with biallelic PCYT2 mutations. The phenotype corresponded to a complex hererditary paraplegia with global DD, regression (4/5), ID (mild in 3/5, severe in 2/5), spastic para-/tetraparesis, epilepsy (5/5 - variable onset 2-16 yrs - focal or tonic-clonic seizures) and progressive cerebral and cerebellar atrophy. Exome sequencing in all revealed biallelic PCYT2 variants, confirmed with Sanger s. in probands and their parents (NM_001184917.2 - corresponding to the canonical transcript used as Ref below): - P1 (Fam1) : 2 missense SNVs in trans configuration, c.730C>T or p.His244Tyr and c.920C>T or p.Pro307Leu - P2 (Fam2 - consanguineous of White British origin), P3 (Fam3 - Consanguineous of Turkish origin), P4,5 (Fam4 - consanguineous, unspecified origin) : homozygosity for c.1129C>T or p.Arg377Ter) affecting the last exon of 8/12 transcripts, including the canonical one. Individuals with the same genotype displayed variable degrees of ID (eg P3 - severe / P2, P4,5 - mild ID). For sibs in Fam4, homozygosity for a missense SACS variant led to consideration of the respective disorder (AR spastic ataxia of Charlevoix-Saguenay) though the variant was predicted to be tolerated in silico and notably the MRI images not suggestive. All variants were absent from / had extremely low AF in public databases, with no homozygotes. Posphatidylethanolamine (PE) is a membrane lipid, particularly enriched in human brain (45% of phospholypid fraction). PE is synthesized either via the CDP-ethanolamine pathway or by decarboxylation of phosphatidylserine in mitochondria. PCYT2 encodes CTP:phosophoethanolamine cytidyltransferase (ET) which is an ubiquitously expressed rate-limiting enzyme for PE biosynthesis in the former pathway. In silico, the 2 missense variants - localizing in the CTP catalytic domain 2 - were predicted to be damaging, as well as to affect protein stability. Fibroblasts of 3 patients (P1, P2, P3) representing all variants were studied: - Enzymatic activity was shown to be significantly reduced (though not absent) compared to controls. Abnormalities were noted upon Western Blot incl. absence in all 3 patients studied of one of the 2 bands normally found in controls (probably representing the longer isoform), reduced intensity in all 3 of another band probably corresponding to a shorter isoform, and presence of an additional band of intermediate molec. mass in patients with the truncating variant. - RT-PCR on mRNA from patient fibroblasts did not reveal (significant) reduction compared to controls. - Lipidomic profile of patient fibroblasts was compatible with the location of the block in the phospholipid biosynthesis pathway and different from controls. The lipidomic profile had similarities with what has been reported for EPT1 deficiency, the enzyme directly downstream of ET. The SELENO1-related phenotype (/EPT1 deficiency) is also highly overlapping. CRISPR-Cas9 was used to generate pcyt2 partial or complete knockout (ko) zebrafish, targeting either the final (ex13) or another exon (ex3) respectively. mRNA expression was shown to be moderately reduced in the first case and severely reduced/absent in the second, compared to wt. Similarly, complete-ko (ex3) led to significantly lower survival, with impaired though somewhat better survival of partial-ko (ex13) zebrafish. Complete knockout of Pcyt2 in mice is embryonically lethal (PMID cited: 17325045) while heterozygous mice develop features of metabolic syndrome (PMID cited: 22764088). Given lethality in knockout zebrafish / mice and the residual activity (15-20%) in patient fibroblasts, the variants reported were thought to be hypomorphic and complete loss of function possibly incompatible with life. PCYT2 is not associated with any phenotype in OMIM/G2P/SysID and not commonly included in gene panels for ID. As a result this gene could included in the ID / epilepsy panels with green (~/>3 indiv/fam/variants with the nonsense found in different populations, consistent phenotype, lipidomics, in silico/in vitro/in vivo evidence) or amber rating. [Please consider inclusion in other possibly relevant panels eg. for metabolic disorders, etc]. Sources: LiteratureKonstantinos Varvagiannis (Other), 11 Nov 2019

Panel Version: 2.1

This gene was added by an external reviewer and rated Green on Hereditary spastic paraplegia gene panel (Version 1.210), and confirmed with Zerin Hyder (Genomics England Clinical Team) that this is appropriate to be Green on the Inborn errors of metabolism panel. The rating of this gene will be changed when the next reiteration of this panel is made. Sources: Literature

Created, Added New Source, Set mode of inheritance, Set publications, Set Phenotypes

Sarah Leigh (Genomics England Curator)

gene: PCYT2 was added
gene: PCYT2 was added to Inborn errors of metabolism. Sources: Literature
Mode of inheritance for gene: PCYT2 was set to BIALLELIC, autosomal or pseudoautosomal
Publications for gene: PCYT2 were set to 31637422; 17325045; 22764088
Phenotypes for gene: PCYT2 were set to Global developmental delay; Developmental regression; Intellectual disability; Spastic paraparesis; Seizures; Spastic tetraparesis; Cerebral atrophy; Cerebellar atrophy
Review for gene: PCYT2 was set to RED

Rating

If promoting or demoting a gene, please provide comments to justify a decision to move it.

Green list criteria

Genes included in a Genomics England gene panel for a rare disease category (green list) should fit the criteria A-E outlined below.

These guidelines were developed as a combination of the ClinGen DEFINITIVE evidence for a causal role of the gene in the disease(a), and the Developmental Disorder Genotype-Phenotype (DDG2P) CONFIRMED DD Gene evidence level(b) (please see the original references provided below for full details). These help provide a guideline for expert reviewers when assessing whether a gene should be on the green or the red list of a panel.

A. There are plausible disease-causing mutations(i) within, affecting or encompassing an interpretable functional region(ii) of this gene identified in multiple (>3) unrelated cases/families with the phenotype(iii).

OR

B. There are plausible disease-causing mutations(i) within, affecting or encompassing cis-regulatory elements convincingly affecting the expression of a single gene identified in multiple (>3) unrelated cases/families with the phenotype(iii).

OR

C. As definitions A or B but in 2 or 3 unrelated cases/families with the phenotype, with the addition of convincing bioinformatic or functional evidence of causation e.g. known inborn error of metabolism with mutation in orthologous gene which is known to have the relevant deficient enzymatic activity in other species; existence of an animal model which recapitulates the human phenotype.

AND

D. Evidence indicates that disease-causing mutations follow a Mendelian pattern of causation appropriate for reporting in a diagnostic setting(iv).

AND

E. No convincing evidence exists or has emerged that contradicts the role of the gene in the specified phenotype.

References

Mode of pathogenicity

Exceptions to loss of function

It’s assumed that loss-of-function variants in this gene can cause the disease/phenotype unless an exception to this rule is known. We would like to collect information regarding exceptions. An example exception is the PCSK9 gene, where loss-of-function variants are not relevant for a hypercholesterolemia phenotype as they are associated with increased LDL-cholesterol uptake via LDLR (PMID: 25911073).

If a curated set of known-pathogenic variants is available for this gene-phenotype, please contact us at [email protected]

Terms

We classify loss-of-function variants as those with the following Sequence Ontology (SO) terms:

Current diagnostic

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in this gene as part of your current diagnostic practice by checking the box

Mode of inheritance

Standardised terms were used to represent the gene-disease mode of inheritance, and were mapped to commonly used terms from the different sources. Below each of the terms is described, along with the equivalent commonly-used terms.

MONOALLELIC, autosomal or pseudoautosomal, not imprinted

A variant on one allele of this gene can cause the disease, and imprinting has not been implicated.

A variant on the maternally-inherited allele of this gene can cause the disease, if the alternate allele is imprinted (function muted).

MONOALLELIC, autosomal or pseudoautosomal, imprinted status unknown

A variant on one allele of this gene can cause the disease. This is the default used for autosomal dominant mode of inheritance where no knowledge of the imprinting status of the gene required to cause the disease is known. Mapped to the following commonly used terms from different sources: autosomal dominant, dominant, AD, DOMINANT.

BIALLELIC, autosomal or pseudoautosomal

A variant on both alleles of this gene is required to cause the disease. Mapped to the following commonly used terms from different sources: autosomal recessive, recessive, AR, RECESSIVE.

BOTH monoallelic and biallelic, autosomal or pseudoautosomal

The disease can be caused by a variant on one or both alleles of this gene. Mapped to the following commonly used terms from different sources: autosomal recessive or autosomal dominant, recessive or dominant, AR/AD, AD/AR, DOMINANT/RECESSIVE, RECESSIVE/DOMINANT.

BOTH monoallelic and biallelic, autosomal or pseudoautosomal (but BIALLELIC mutations cause a more SEVERE disease form), autosomal or pseudoautosomal

A variant on one allele of this gene can cause the disease, however a variant on both alleles of this gene can result in a more severe form of the disease/phenotype.

A variant in this gene can cause the disease in males as they have one X-chromosome allele, whereas a variant on both X-chromosome alleles is required to cause the disease in females. Mapped to the following commonly used term from different sources: X-linked recessive.

X linked: hemizygous mutation in males, monoallelic mutations in females may cause disease (may be less severe, later onset than males)

A variant in this gene can cause the disease in males as they have one X-chromosome allele. A variant on one allele of this gene may also cause the disease in females, though the disease/phenotype may be less severe and may have a later-onset than is seen in males. X-linked inactivation and mosaicism in different tissues complicate whether a female presents with the disease, and can change over their lifetime. This term is the default setting used for X-linked genes, where it is not known definitately whether females require a variant on each allele of this gene in order to be affected. Mapped to the following commonly used terms from different sources: X-linked dominant, x-linked, X-LINKED, X-linked.

MITOCHONDRIAL

The gene is in the mitochondrial genome and variants within this can cause this disease, maternally inherited. Mapped to the following commonly used term from different sources: Mitochondrial.

Unknown

Mapped to the following commonly used terms from different sources: Unknown, NA, information not provided.

Other - please specify in evaluation comments

For example, if the mode of inheritance is digenic, please indicate this in the comments and which other gene is involved.